1. Field
The present disclosure relates to the field of optical communication technologies, and in particular to an apparatus for monitoring an optical signal to noise ratio, a transmitter and a communication system.
2. Description of the Related Art
An optical signal to noise ratio (OSNR) is a quantity directly associated with performance of the system both in a conventional direct detection optical communication system and a coherent optical communication system. Hence, much attention has been paid to the study of optical signal to noise ratio monitoring technologies.
In a conventional communication system, the optical signal to noise ratio may be obtained by analyzing a spectrum of an optical signal. For a wavelength division multiplexing (WDM) system containing multiple wavelengths, signal power and noise power of a present wavelength may be obtained by switching signal light of a present channel. It is obvious that such a monitoring method interrupts traffic transmission. In order that traffics are ensured while performing OSNR monitoring, the conventional method needs to assume that a spectrum of a noise is flat and at the same time, a bandwidth of a transmitted signal is far less than a channel interval, hence, it may be selected to measure the signal and the noise at different frequency points, so as to obtain relatively accurate estimation of the OSNR.
However, as increase of an optical communication capacity, a transmission length and transmission rate of a coherent optical communication system are greatly improved than before. More optical nodes will result in larger fluctuation of spectrum shapes of noises, and an assumption that noises are evenly distributed in the spectrum will face larger challenges. And at the same time, as the channel intervals are greatly reduced, finding a band in which signals may be neglected to measure noise power becomes an impractical subject. Thus, measurement of an OSNR in a coherent communication system becomes new hot spot of studies.
In a coherent communication system, there are some methods using received light-field signals to estimate an OSNR. Transmission signals containing noises may be recovered from the received signals after the received signals experience a series of digital signal processing algorithms, such as sequence synchronization, dispersion compensation, equalization, frequency difference removal, and corresponding phase noise removal, etc. Assuming that a transmission sequence (a training sequence) is known, or an approximate transmission sequence is obtained by direct judgment, sizes of the signal and the noise may be obtained respectively, thereby estimating the OSNR of the system[1].
Nippon Telegraph & Telephone (NTT) proposes an OSNR estimation method based on pilot signals[2], in which a training sequence is designed into a form of single frequency, and power of the single frequency and a power spectrum density of noises around it may be obtained at a receiver end by using a detection algorithm, thereby obtaining an OSNR of an optical signal of the channel.
It should be noted that the above description of the background is merely provided for clear and complete explanation of the present disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of the present disclosure.
Documents advantageous to the understanding of the present disclosure and the relevant art are listed below, which are incorporated herein by reference, as they are fully described herein.
[1] IEEE Photonic Journals 2013, pp. 6601609; and
[2] OECC 2013, TuR2-4, Digital In-band OSNR Estimation for Polarization-Multiplexed Optical Transmission.